Power Tips: Bi-Directional Power in a Dual Battery 12V/48V Automotive Design

[MUSIC PLAYING] Greetings. I'm Dave Priscak with Texas Instruments, your host for the Power Tips Ask the Expert video series. I'm here in Freising, Germany today with Roberto Scibilia, who will be talking to us about the 48 volt bus structure in today's automobiles. Roberto, welcome.
Thank you, Dave. The new 48-volt bus system is driven mainly by the CO2 emissions reductions, as well as the high-demanding currents on the existing 12-volt battery. And mainly, the five OEMs car manufacturer are driving this kind of a new bus, which are, Daimler, for example, BMW, and Porsche, and so on. They are also taking into account that new standard, the new specification, LV 148.
If we see, in the first slide, we can understand how many loads are increasing the power on the 12-volts, and therefore, we would need the higher currents and bigger conductors to [INAUDIBLE] inside a car. This will be pretty bad, in terms of cost and in terms of weight. Therefore, that new 40-volt bus system takes into account, actually, a 48-volt lithium-ion battery.
And we can see that, for example, the three-phase driver is just a system that recovers the energy from the braking and also keeps the car moving, actually. And we need, in this case, a bi-directional power supply for a simple reason, just to make the two batteries alive in any condition, so you have always a source of the power in your car. And what will be a specification for this kind of new bi-directional power supply? Mainly, the power level is between 400 watts and three kilowatts, I would say, with a two phase, up to eight phases.
So when you were talking about bi-directional, that's because the 12-volt system is going to remain.
Exactly.
This is an additional 48-volt bus to today's 12-volt system.
Exactly. The car will stay almost the same. We will have second source of energy on the 48-volt lithium-ion battery. And the efficiency is very important, of course, because we are targeting 97% 98% of efficiency, to avoid complicated or very, very expensive cooling system, as well as the ability to withstand, with the 70-volts, a load dump on the 40-volts lithium-ion battery.
But what is actually bi-directional power supply? You can build a bi-directional power supply with a simple buck converter. We can draw a very, very simple schematic here. We have a first battery. You have, you know, perfectly how does it work. Is just a buck converter with a LC filter and with two terminals on the output. We have L. We have C. We have VB1. It's the first battery.
Now, if you modulate the two switches in a normal way, in continuous conduction mode, like this way, the peak will be VB1. And the average of this waveform will be Vout, which we call Veq here, Veq. Now, we can replace this box with a equivalent generator, which is this one, Veq. And it has also an equivalent impedance, Veq. Now, we connect a second battery here. In this case is a 12-volt battery. And we call it VB2. VB2 will be connected on the output, V2.
Now we understand. Very simple. The current flowing from left to the right, this current, is the output current of this power supply. And then, this depends on the delta V, which is the voltage here. So this delta V equal to Veq minus VB2, divide by the impedance.
Now, you understand that the small delta V on this schematic draws the current from left to the right, or a smaller Veq than VB2 absorbs current from the second battery. In this case, we have a bi-directional power supply. Very simple.
So in theory, we can use just a normal controller for a battery power supply. This is not possible, because we need to sense the current on the inductor here and to push this current in our controller. The controller now will modulate the MOSFETs.
Since we need to manage this bi-directional current, we can think about, in the new slide, about a new architecture. The new architecture is then is done by several modules in parallel with the 48 and 12-volts battery. And we usually take one module in two-phase architecture. And we can use special, let's say, drivers and microcontrollers together to think about only the analog or digital mixed architecture.
So when you talk about modules, you're talking about each with two phases, for example.
Correct.
And so you would size the number of modules, based upon the amount of power transfer you sense.
Correct. Since we though about 400 watts up to three kilowatts nominal power, we can put several modules in parallel, up to eight phases.
Now you can see, from the slide, that we have an inner current loop, which is a fast loop, and then outer current loop. The outer current loop is a very slow loop, because it must only watch the output voltage, which are batteries. Batteries are changing the voltage very, very slowly. So we need a very fast current loop and very slow voltage loop. The third microcontroller will be really needed to interface my power supply to the car system.
TI is working on three approaches. The first one is completely digital with C2000, the Piccolo BF2869 plus driver. And the second approach is with LM5170, which is a completely analog solution with a current source, which is pushing or absorbing current from one battery to another battery. And it works also in interleave mode, so it has two phase. And you still need a microcontroller, low computational power, to interface with the actual system.
And the third approach will be with the UCD3138. Here, you need just standard drivers. And the difference from the other two approaches is that we have here, inside the microcontroller, the current loops, which do not need any firmware effort. So you can write your firmware, without thinking about the current loops. Still, a microcontroller is needed in this case to interface with actual system.
I see. So the C2000 is purely digital. So you have total control over not only the power transfer, but the loop itself.
Correct.
And so when you're talking about two different lithographies of batteries, for example, so two different charge areas, digital might make sense. But on the far right, we have the analog. If it's just sheer power transfer, the analog system will probably work better, because it's a simple transfer of energy, correct?
Correct.
OK.
So depending on the needs, on how much you have experience on the digital part, compared to the analog part, that will be better, maybe, to choose the analog solution. Instead, if you prefer to have your own IP to make everything by firmware on the C2000 part, then you can write your own code and avoid any analog block.
So Roberto, we've talked about the three different methodologies TI has for this power conversion. Can you show us an example?
Yes. This is a prototype. It's a two-phase, 700 on watts power. And it is based on the Piccolo B C2000 microcontroller. This is a control stick. And it is completely digital, so there is no analog part inside. And here, we can see two drivers, SM72295.
And it's a 700-watt system you say?
Yes. Correct.
Thank you very much, Roberto. For more information, please visit ti.com. And stay tuned for more videos in the Power Tips Ask the Expert video series. Thank you, very much. Thank you, Roberto.
Thank you.
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Details

Date:
December 31, 2015

In this Power Tips "Ask the Expert" video Dave asks TI’s Roberto Scibilia how to manage bi-directional power in a dual-battery 12V/48V automotive design. Roberto explains how to control power to the steering control and ADAS applications, and how to create and manage a connection between 12V and 48V.